Technical Field
The present subject matter relates to the field of co-operative systems and especially a method to communicate within a co-operative system.
Background Art
A co-operative system comprises a plurality of communication units, which communicates in that they broadcast on a given wireless communication channel designated for the co-operative system. The communication units thereby broadcast information and receive information. The communication units can be integrated in vehicles and/or infrastructure in order to let the devices communicate with each other.
Co-operative systems are used as traffic information systems to facilitate vehicle-vehicle and vehicle-infrastructure communication.
One of the challenges in a co-operative system is that of the distribution information, i.e., the vehicle data that shall be broadcasted from every vehicle within a set interval.
For example, the vehicles transmit their data once every 100 milliseconds (ms). The data is typically a time-stamped position, speed and heading of the vehicle, but can further comprise information from vehicle sensing means such as speed sensing means, acceleration sensing means, road conditions sensing means, weather condition sensing means, driver intention sensing means, etc.
Normally the raw data is collected by a raw data collector, such as a global navigation satellite system (GNSS), or any other sensing means within a vehicle, such as speed sensing means, acceleration sensing means, road conditions sensing means, whether condition sensing means, driver intention sensing means, etc.
The raw data is computed into an information package that can be broadcasted within the co-operative system. E.g., a GNSS receiver continuously receives a position signal from the GNSS, wherein this information is acquired by the communication unit, at the same time for each of the devices in the co-operative system. The information is fed into a position solution engine, which typically is a Kalman filter. The position solution is computed into the information package within fractions of a full second. The information package can include further information collected by other raw data collectors, whereby the information package thereby includes also this information.
The time that it takes to compute the information package is not possible to predict precisely since it is dependent on many factors, such as amount of data and current speed of the device. Hence, there is a small range of time when the output from the position engine is given. When the output is ready, the data is included in a certain information package dedicated for broadcast in a co-operative system. This message is referred to as “Basic Safety Message” in the US and “Co-operative Awareness Message” in Europe, but will in the further text be referred to as information package.
As soon as the information package is ready, the radio module tries to access the wireless communication channel of the co-operative system and broadcast the information package. Because all devices acquire their data at a start of an interval and perform similar computations, there is an obvious risk that several devices try to access the channel to broadcast their respective information packages at the same time, which will result in that some transmitters have to back off and try again later. The back-off and retry scheme is controlled by algorithms in the 802.11 standard and is outside the scope of the present subject matter.
In order to improve the accessibility to the communication channel, several different methods have been proposed to distribute the time on where the transmitters try to access the wireless communication channel. One of those is called “Self Organized Time Division Multiple Access” where the devices are organized to transmit in a certain time slot, which is defined in an overall master frame. The master frame is distributed among the users without the interaction of a central station.
When using “Self Organize Time Division Multiple Access” for example within a system with three devices, the devices are getting their information packages transmitted at a pre-defined time, but there is always a delay in time introduced between the finish of the computation, i.e., when the information package is ready to be sent, and the broadcast of the information package. When the information package is actually broadcasted, the information package data in the information package has aged during the delay.
By communicating aged data in the co-operative system, errors can occur and the relevance of the information in the system is decreased.